Breast Cancer Pregnancy
Credit: Copyright 2018 Vera Lair/Stocksy

There’s a lot we don’t know about breast cancer — from the kinds of environmental factors that make a diagnosis more likely to what you should do if you find out you’re BRCA positive, much of the disease is still a mystery. But some new research seems to confirm one thing definitely reduces your risk of getting breast cancer: pregnancy — specifically, how long your pregnancy lasts.

New research out of Denmark looking into the birth and health records of nearly 4 million Danish and Norwegian women found that women who carried a pregnancy to at least 34 weeks had a smaller chance of ever being diagnosed with breast cancer — it reduced their risk by about 14 percent. And, each pregnancy that lasts that long helps: If a woman had three pregnancies of at least 34 weeks, her lifetime risk dropped by nearly 40 percent. By comparison, a pregnancy at 33 weeks only reduced a woman’s lifetime breast cancer risk by 2.4 percent. That is a major difference. What’s happening?

“It's not completely understood, but one theory is that the normal breast cell changes that occur during and after pregnancy protect the breasts from breast cancer,” says Jennifer Conti, MD, a clinical assistant professor of obstetrics and gynecology at Stanford and co-host of The V Word podcast. And so it makes sense that the further along in a pregnancy you get, the better this mechanism might work, although this study indicates there seems to be some magic happening at 34 weeks — additional weeks beyond that benchmark didn’t make a difference. What that is, we don’t yet know.

This fits in with what we already know about how pregnancy helps reduce breast cancer risk, which is based on the idea that having fewer menstrual cycles over the course of your life reduces your total exposure to the ovarian hormones that make breast cancer more likely. (That’s also the theory as to why breastfeeding is also correlated with lower risk, since it often suppresses your period.) But this research is the first to look at the length of a pregnancy as a key part of that reduced risk. In terms of the protective effect for the mother, it didn’t matter when she went into labor or even if the birth was a stillbirth, as long as it was after 34 weeks (of course, there are many other risks for preterm babies, and a baby isn’t considered full term until 37 weeks). Further complicating things, some of the factors associated with preterm birth — like obesity and being over 35 when you have your first baby — may be linked to breast cancer, so it’s hard to know exactly what’s at work here.

“When we talk about breast cancer risk as it pertains to pregnancy, we need to keep in mind one huge factor, which is that we often don't have control over these situations,” says Dr. Conti. This is especially true when it comes to fertility — we can’t will ourselves to carry a pregnancy to 34 weeks or more, or have a baby before age 35. Besides, “nobody decides to get pregnant and have a child just to decrease their breast cancer risk, nor should they. There are many other risk factors, like smoking, obesity, and alcohol use, that also increase risk and that we can control. These are the things we should focus on improving,” she says.